1. The Field of the Invention
This application relates to performance and reliability testing of laser diodes and other semiconductor lasers.
2. The Relevant Technology
Laser diodes and other semiconductor lasers are an essential component of many current technologies. In many applications, such as optical network devices, the lasers must operate with high reliability in order to provide continuous operation while minimizing maintenance time and expense.
One common characteristic of many semiconductor lasers is a particular shape for the lifetime curve of the device. Many semiconductor lasers exhibit a ‘bathtub’ curve for average lifetime, as shown in FIG. 1. The bathtub curve shape arises for devices that have a high ‘infant mortality’ rate, where a substantial number of devices fail after a relatively short period of use due to manufacturing defects.
As a response to the bathtub shaped failure curve, semiconductor lasers are typically subjected to performance testing in the form of a ‘burn-in’ procedure. During a burn-in procedure, the laser is operated at a specific current and temperature for a fixed duration. The laser is then tested to see if it is still performing within desired specifications. Performing burn-in allows flawed devices to be identified before they are incorporated into larger product assemblies or shipped to customers. This leads to increased reliability for the remaining lasers that are incorporated into products in the field. The operating conditions for a burn-in procedure are selected to identify lasers that exhibit early failure while minimizing the stress on the other properly manufactured lasers. The operating conditions for a burn-in procedure are typically developed by trial and error, and finding a desirable set of operating conditions may take up to a year of testing or longer. A key consideration in developing burn-in procedures is to find the operating conditions that optimize the burn-in time or cost while still capturing all of the units that would fail burn-in. That is, a goal is to minimize the time and/or expense necessary to get to point t0 on the bathtub curve shown in FIG. 1, where further testing does not identify any additional early failing units.
Another common form of performance testing is wafer qualification. For lasers with high infant mortality rates, such as most laser diodes, every device manufactured will be subjected to burn-in, as the goal is to identify flawed devices. Wafer qualification, on the other hand, is performed on a small sampling of the total devices produced. During wafer qualification, one or more representative die from a processed wafer are placed in a test apparatus for extended life testing of the device. The sample devices are tested to verify that no variations have occurred in the manufacturing process that would lead to impaired performance for other devices in same production batch. Developing appropriate operating conditions for wafer qualification is also a time consuming process.
Because of the difficulties in developing operating conditions for performance tests, device manufacturers typically develop a minimal number of operating conditions for each type of performance test. This limits flexibility during the manufacturing process, as these time consuming and costly procedures cannot be optimized to improve productivity.
One method for collecting additional information about desirable operating conditions for performance testing, including burn-in and wafer qualification, is via “Life Data Analysis.” Traditional “Life Data Analysis” involves analysis of time-to-failure data obtained under selected operating conditions in order to quantify the life characteristics of the product. Such life data, however, is difficult to obtain for products with long expected lifetimes. This difficulty, combined with the need to observe failures of products to better understand their life characteristics, has led to the development of methods to force such long-lived products to fail more quickly. In other words, these methods attempt to accelerate the failure rate through the use of high stress conditions.
Analysis of data from such accelerated life testing can yield valuable information regarding product life as a function of design conditions, but only if the ‘accelerated’ conditions of the failure test can be properly correlated with operating conditions during normal use. In order to correlate the ‘accelerated’ and typical operating conditions, the accelerated life must incorporate the correct variable dependencies, such as the dependence of laser lifetime on the junction temperature of the laser diode.
Characterizing expected device lifetimes is also becoming increasingly important for the development and characterization of end-user products. For example, previous generations of optical networks often made use of transceivers that were cooled to maintain desired operating conditions. Newer generations of this technology, however, are moving toward uncooled transceivers. Due to the high reliability requirements of optical networks, understanding device lifetimes is important both for identifying the level of redundancy required in the system as well as for developing maintenance schedules.
Therefore, there is a need for an improved method of establishing operating conditions for burn-in, wafer qualification, and other performance test procedures. The method should facilitate identification of equivalent performance test operating conditions once baseline conditions have been established empirically. The method should incorporate life data from accelerated life testing for developing procedure parameters. The method should allow for correlation of performance testing conditions with typical device operating conditions in one or more desired applications. Additionally, the method should allow for characterization of device lifetimes under various operating conditions in order to assist with the design and maintenance of applications involving the device.